U.S. Pat. No. 5,155,599 “Screening system and method for color reproduction in offset printing” describes a concept for generating sets of at least three halftone screens that are allegedly free from second order moiré. The concept is targeted at offset printing and flexographic printing systems and is discussed in the context of binary halftoning.
U.S. Pat. No. 5,155,599 deals with the optimal arrangement of at least three separation preangled screens in a supercell. The screening angles that are used are close, but not identical to conventional screening angles of 15 degrees, 45 degrees and 75 degrees. The reproduction is nevertheless free of second order moiré by the fact that the deviations in angles from the conventional system are exactly offset by the deviations in line rulings.
U.S. Pat. No. 5,155,599 is incorporated by reference in its entirety.
The concept of generating one of the prescreened tiles is briefly indicated using FIG. 1.
The screen angle and the spatial frequency of the dot modulation of the angled screens—also referred to as screen ruling—is expressed in the units of lpi (lines per inch) and is derived asalfa=a tan(A/B)F=res*sqrt(A2+B2)/TS Where “res” is the spatial resolution of the imagesetter, which has evolved from 2400 dpi (dots per inch) in the 90's to over 4000 dpi presently.
The average number of recorder elements available for building each screen dot equals (Res/F)2. Due to the binary nature of the printing system and the requirement of more than 100, preferentially more than 200 graylevels, available the screen Res/F should be higher than 12 or more preferentially higher than 20 in order to be able to render 144 and 400 distinct levels respectively. Typically 256 levels are required by many printing applications and is a de facto industry standard. Preferably an excess of that should be available to allow tone curve adjustment. An example for 2400 dpi imagesetters is therefore, typically around 120-140 lpi.
Digital printing systems, such as electrographic digital printing systems and ink jet based digital printing systems may have a more restricted spatial resolution of less than 2400 dpi or 3600 dpi such as for example 600 dpi or 800 dpi or even 1200 dpi.
Modern expectations for screens in high quality printing are to have the option to select screen rulings of 200 lpi. Methods used in offset for generating high resolution screens cannot be readily carried over to digital print technologies, however, as 10-15 micron features cannot be stably printed in these technologies at the present time.
Whereas the offset printing system is binary—either there is a laydown of ink or there is no laydown of ink—for a given addressable position on the print medium, these digital printing systems may be capable of rendering multiple density levels for each addressable position on the print medium. The typically lower device resolution is compensated for to a certain extent by the multiple density resolution capability.
There remains a need for screening concepts that are specifically tuned to the capabilities and limitations of digital printing systems such as digital printing systems that are based on electrophotography.
FIG. 2 shows a simplified representation of how a clustered dot screen rendering for a given input grayscale value in binary printing at a higher device resolution (12), can be converted to a lower resolution (13) representation with multiple density capability at the device pixel level. The screen ruling in FIG. 2 is indicated by the reference number 11.
FIG. 3 shows an example of how a prescreened tile with A=2, B=2, TS=11 can be generated according to the principle of FIG. 2, which is sometimes referred to as a box-filter. A square screen tile (40) comprises 8 clustered dots in a square arrangement with a screen angle of 45 degrees. Target dot centers (15) are indicated by circles. The device grid with resolution (13) is indicated by the grid lines. Target dot centers 15,16 and 17 have a different relative position with respect to the intersections of the device grid lines.
FIG. 4 shows a pattern as obtained after screening a uniform grayscale area with a grayscale value of 80% for a box filter approach of the geometry of FIG. 3 (100% being white, 0% being black). The repeating screen tile 40 is indicated in the representation of the screened image in FIG. 4. Note that clustered dot configurations 15b, 16b and 17b are quite different in terms of the occurrence of different values for the density levels of the contributing pixels.
It has been found from experiments with electrographic digital printing systems that the density contribution of dot configurations such as 17b is far less stable than the density contribution of dot configurations such as 16b. 
Especially electrophotographic digital printing systems using toner development assisted by a superimposed alternating electric field (AC) such as the Canon Imagepress V7000 and the dual component (AC) assisted development in the Xeikon 5000 and Xeikon 6000 printers have a highly non-linear development process. Donor roll development, as used in the Xerox Igen3 digital press, also has a highly non-linear development process.
It is claimed that the rotating magnetic brush development system utilized in the Nexpress 2100 system and the Nexpress S3000 system has an intrinsic capability to develop a continuous tone representation without introducing a screen—see FIG. 13 and the discussion on page 491 of Satellite images in “Advances in Technology of KODAK NEXPRESS Digital Production Presses” in NIP23 and Digital Fabrication 2007, pages 489-493 published in 2007 by IS&T (ISBN 0-89208-273-9).
The Nexpress 2100 system has typically been using screens for its black printing as in FIG. 3, where the screening method can be similar in grayscale characteristics to that of a simple box-filter. The most popular screen that is made available for printing the black separation on the Nexpress 2100 is the 155 lpi screen with a 45 degrees screen angle and corresponds more or less to the configuration of FIG. 3 for a device resolution (13) of 600 dpi.
Box filters calculations are lightweight and can be used for screen calculation on the fly as with irrational tangent screening.
Experiments have shown that screens as in FIG. 3 give rise to poor results in electrophotographic digital printers that utilize alternating current bias assisted dual component magnetic brush development. The density contribution of dot configurations such as 17b in FIG. 4 are found to vary significantly with minor changes of the development setup that can result from environmental changes, wear of components, cleanliness of engine parts and the aging of the consumables. As the varying of the density contribution of dot configurations such as (16b) is different and generally far less than the varying of the density contribution of the dot configuration (17b) such changes in the print conditions will generally give rise to observable patterns at low spatial frequencies that can ultimately get as low as the repeat frequency of the entire screen tile (40) (55 lpi for FIG. 3 at 600 dpi device resolution). Whereas the eye does not pick up the 155 lpi modulation from normal viewing distances, the lower frequency patterns resulting from unwanted 55 lpi modulation are easy to pick up by an observer.
As these low frequency patterns result from how the cluster dot formation is affected by the device grid, this type of unwanted low frequency modulation will be referred to as due to an intrinsic internal moiré effect that is amplified by process instability.
Similar issues of amplification of an intrinsic internal moiré can be expected with donor roll development where thin wires in the development gap induce AC fields that lead to a steep and non-linear development curve as is used in the Igen3 model of Xerox Corporation. Liquid toner development is also known to have a nonlinear development curve.
Approaches to minimize and control the use of “unstable levels” in the design of multilevel screening algorithms have been discussed in amongst others U.S. Pat. Nos. 5,444,551, 5,903,713 and 5,654,808.
Earlier attempts to take the concept of U.S. Pat. No. 5,155,599 to digital printing systems have been only partly successful. The Xeikon DCP family including the more recent Xeikon 6000 printing system has been providing 170 lpi on a 600 dpi device with the black screen under 45 degrees being sensitive to an intrinsic internal moiré effect that is amplified by process instability with a resulting frequency at 120 lpi.
Japanese copier and printer manufacturers have focused on the use of simple rational screens with a small repeat cell including sets with a black separation at 212 lpi, 45 degrees and additional separations under screen angles of 18 degrees and 72 degrees. Such screens have a small rectangular repeat structure for the overprint of the three separations. The images lack the symmetry of the conventional “rosette structure” which is obtained when the screen angles of the separations screens approach the 30 degrees rotation that is typical for screensets that approach the angles of a conventional set of 30 degrees rotated screens.
It has now been found that the perception of overprint patterns is highly reduced when the screen ruling is increased. For a given screen ruling a further reduction in the perceived level of overprint patterns was found the more the classical isotropic “rosette structure” as known from conventional 30 degrees rotated clustered dot screens is approached.
Screening approaches as in the HP Indigo 5500 are based on square screen tiles. Oversized dots are used and highlight areas are screened using large square screen tiles with randomized dot positions. Screen configurations in the midtones approach the conventional dot structure of 30 degrees rotated screens. The rosette structure evolves from a clear centered to a dot centered within a page indicating that the rosette is shifting and not locked in the terminology of U.S. Pat. No. 5,155,599.